The present invention relates to an intruder detector system and, in particular, it concerns a system which scans an area with a laser beam and then analyzes the reflected light to determine if an intruder has entered the area.
Security is becoming an increasingly complicated issue in today's world. Even one of the most basic threats, that of an intruder attempting to gain access to a building or other protected area, has become much more difficult to counter. As the technology to protect these secure areas has become more sophisticated, so have those criminal individuals who wish to subvert it. Thus, there is an increasing need for sophisticated systems to detect the presence of these intruders in protected areas.
One example of an attempt to solve this problem is a method of intruder detection which is disclosed in U.S. Pat. No. 4,949,074. In this method, a beam of infrared radiation is projected and the reflected radiation is measured. However, this method does not include the ability to scan a large area or to track a moving intruder. Indeed, the figures and examples in U.S. Pat. No. 4,949,074 show only narrow rectangular corridors, rather than wide, irregularly shaped areas. Furthermore, the range of an infrared beam is also relatively short. Thus, the method of U.S. Pat. No. 4,949,074 is highly limited in potential applications.
Similarly, U.S. Pat. No. 3,715,953 disclosed an serial surveillance and fire-control system which includes a laser rangefinder. However, the rangefinder is used only to aim at a target, and does not actively scan the area under surveillance.
Clearly, being able to scan an entire area would enable intruder detection systems to survey much larger, wider or irregularly shaped areas. Such scanners are also called rangefinders, since they measure the distance between the object and the rangefinder itself. U.S. Pat. No. 4,939,379 discloses a system for actively scanning the surface of an object and measuring its contours. However, this system is designed to be used to scan stationary objects, and has no provisions for detecting or tracking moving objects.
Another example of a range finder is disclosed in U.S. Pat. No. 4,788,441. This range finder includes a light source at a first location, a retroreflector attached to the target at a second location and a photoreceptor. A beam from the light source can be scanned along a path and is then reflected back by the retroreflector. The photoreceptor receives the reflected light. The distance between the light source and the retroreflector is then determined. Without the retroreflector, the device in inoperable. The range finder of U.S. Pat. No. 4,788,441 is thus limited to those situations where a retroreflector can be attached to the target.
However, such a limitation implies the compliance of the target, or of the individual operating the target, with being monitored, Certainly, such compliance would not be available from an intruder attempting to gain access to a protected area, since theses individuals are actually attempting to avoid detection. Thus, clearly a system is required which can also scan a light beam from a light source along a path, collect the reflected light and then analyze it, without requiring a retroreflector. Such a system would be able to detect and analyze light reflected from any objects in the area to be scanned, including human beings.
There is therefore a need for, and it would be highly useful to have, a system for detecting the presence of an intruder in a protected area, which scans the area with alight beam and then analyzes the reflected light to determine if such an individual is present.